The present invention relates to a radiation image storage panel employable in a process for reading a radiation image in the radiation image recording and reproducing methods, by a single- or double-side reading system.
As a method replacing a conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed, and is practically employed. The method employs a radiation image storage panel (i.e., stimulable phosphor sheet) comprising a stimulable phosphor, and comprises the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as xe2x80x9cstimulating raysxe2x80x9d) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The panel thus treated is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly employed.
In this method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at a considerably smaller dose, as compared with a conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the method is very advantageous from the viewpoints of conservation of resource and economic efficiency because the radiation image storage panel can be repeatedly used while the radiographic film is consumed for each radiographic process in the conventional radiography.
The radiation image storage panel has a basic structure comprising a support and a stimulable phosphor layer provided thereon. However, if the stimulable phosphor layer is self-supporting, the support may be omitted. On the free surface (surface not facing the support) of the phosphor layer, a protective film is generally placed to keep the phosphor layer from chemical deterioration or physical shock.
The stimulable phosphor layer basically comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor particles without binder. The stimulable phosphor layer containing no binder can be formed by deposition process or firing process. Further, the stimulable phosphor layer comprising agglomerated phosphor soaked with a polymer is also known. In any of stimulable phosphor layers, the stimulable phosphor emits stimulated emission when excited with stimulating rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the phosphor layer of the radiation image storage panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the storage panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the storage panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recorded in the storage panel is generally read by the steps of irradiating stimulating rays on the front surface side (phosphor layer side) of the panel, collecting light emitted by the phosphor particles by means of a light-collecting means from the same side, and photoelectrically converting the light into image signals. A system for reading the image from one side of the panel in this manner is referred to as xe2x80x9csingle-side reading systemxe2x80x9d. However, there is a case that the light emitted by the stimulable phosphor particles should be collected from both sides (i.e., from the front and the back surface sides) of the storage panel. For instance, there is a case that the emitted light is desired to be collected as much as possible. There is also a case that the radiation image recorded in the phosphor layer varies along the depth of the phosphor layer, and that detection of the variation is desired. A system for reading the image from both sides of the storage panel is referred to as xe2x80x9cdouble-side reading systemxe2x80x9d.
A radiation image storage panel employed in the double-side reading system, as well as a radiation image storage panel employed in the single-side reading system, is desired to be as sensitive as possible and to provide an image of high quality (high sharpness, high graininess, etc.).
While repeatedly used (particularly, while repeatedly transferred for reading and erasing in the reading system), static electricity is deposited on the radiation image storage panel. Since the deposited static electricity gives image artifacts, the storage panel having static electricity thereon is apt to give an reproduced radiation image of poor quality. For avoiding the impairment with electrification, it has been proposed to incorporate various antistatic agents into the storage panels for single-side reading system. For example, Japanese Patent Publication H6-31911 describes a radiation image storage panel having at least a member containing fibrous electroconductive material (mean length: 5 to 50 xcexcm, mean diameter: 0.1 to 1.0 xcexcm), and Japanese Patent Provisional Publication H4-2999 also discloses a radiation image storage panel containing electroconductive zinc oxide whiskers (mean length: 3 to 150 xcexcm, mean diameter: 0.3 to 3 xcexcm).
The panel for single-side reading system having a protective film containing conventional conductive material (such as zinc oxide whiskers) is satisfactory from the viewpoint of antistatic property. However, it often gives an image of very poor quality because the conductive material absorbs a portion of the stimulated emission. For example, a europium activated alkaline earth metal fluoride halide phosphor (which is a representative stimulable phosphor) emits stimulated emission having a peak at approx. 400 nm, while the conventional conductive material absorbs light in the shorter wavelength part of that stimulated emission. Consequently, the amount of the observed emission decreases to impair the resultant image quality. Particularly in a radiation image storage panel for double-side reading system, the conductive material contained in any layer or film reduces the amount of the observed emission to seriously impair the image quality.
It is an object of the present invention to provide a radiation image storage panel for the use in the single- or double-side reading system. The target radiation image storage panel has an excellent antistatic property and gives a reproduced radiation image of high quality.
The present inventor has studied about the conventional antistatic agents such as zinc oxide whiskers, and finally has found the following fact. The conventional antistatic agent absorbs light in the shorter wavelength part of the stimulated emission, and accordingly reduces the amount of the detectable emission to impair the quality of the image given by the storage panel for single- or double-side reading system. To solve this problem, the inventor further studied and found that an antistatic agent which was not employed in the radiation image storage panel ensures the amount of the detectable emission. The antistatic agent comprises electroconductive acicular fine particles having major and minor axes shorter than those of the conventional one, and hence less absorbs the light in the emission wavelength than the conventional one. Accordingly, it can effectively reduce the surface electric resistivity of the radiation image storage panel without lowering the transmittance of the stimulated emission.
The present invention resides in a radiation image storage panel comprising a stimulable phosphor layer which gives stimulated emission upon irradiation with stimulating rays and a protective film provided thereon, wherein the protective film contains electroconductive acicular fine particles, has a surface electric resistivity of 1014xcexa9 or less, and shows a transmittance of 70% or more at a peak wavelength of the stimulated emission.
The invention further resides in a radiation image storage panel used for a double-side reading system, comprising a back protective film, a transparent support film, an undercoating layer, a stimulable phosphor layer which gives stimulated emission upon irradiation with stimulating rays, and a protective film overlaid in order, wherein at least one layer or film selected from the group consisting of the back protective film, the undercoating layer and the protective film contains electroconductive acicular fine particles, has a surface electric resistivity of 1014xcexa9 or less, and shows a transmittance of 70% or more at a peak wavelength of the stimulated emission.
In the invention, the transmittance of the layer or film containing the acicular fine particles is the total transmittance including loss caused by surface reflection. In the present specification, the xe2x80x9cfront surfacexe2x80x9d means the top surface of the stimulable phosphor layer (if a protective film is provided on the phosphor layer, it means the surface of that protective film). The xe2x80x9cback surfacexe2x80x9d means the bottom surface of the phosphor layer (if a support is provided, it means the bottom surface of that support, and further, if a back protective film is provided, it means the bottom surface of that film).